Value metal base electrode coated with pb2ru2o6 or pb2ir2o6

ABSTRACT

ELECTROLYSIS OF BRINES IN ACCOMPLISHED WITH ANODES HAVING AN OPERATIVE SURFACE OF LEAD RUTHENATE OR LEAD IRIDATE. THE ANODES ARE PARTICULARLY EFFECTIVE FOR THE PRODUCTION OF CHLORINE. IT HAS BEEN FOUND THAT LEAD RUTHENATE OR LEAD IRIDATE POWDER MAY BE BONDED TO A VALUE METAL SUBSTRATE BY USING A LOW MELTING GLASS FLUX. ONE GROUP OF GLASSES EXISTS IN THE V2O5PBO SYSTEM, CONSISTING OF V2O5 PLUS ABOUT 30 TO 55 MOL PERCENT OF PBO. OTHER TYPES OF FLUXES ARE LEAD BOROSILICATE GLASS OR LEAD BORATE: BISMUTH SUBNITRATE IN THE PROPORTION OF 2:1 PARTS BY WEIGHT.

United States Patent Oflice 3,691,052 VALUE METAL BASE ELECTRODE COATEDPbzRllgO Pb II' OS Robert C. Langley, Millington, N.J., assignor toEngelhard Minerals & Chemicals Corporation No Drawing. Filed Jan. 7,1971, Ser. No. 104,797 Int. Cl. B011 3/04 US. Cl. 204-290 F 6 ClaimsABSTRACT OF THE DISCLOSURE Electrolysis of brines is accomplished withanodes having an operative surface of lead ruthenate or lead iridate.The anodes are particularly effective for the production of chlorine. Ithas been found that lead ruthenate or lead iridate powder may be bondedto a value metal substrate by using a low melting glass flux. One groupof glasses exists in the V O .PbO system, consisting of V plus about 30to 55 mol percent of PhD. Other types of fluxes are lead borosilicateglass or lead boraterbismuth subnitrate in the proportion of 2:1 partsby weight.

BACKGROUND OF THE INVENTION This invention relates to novel anodes forthe electrolysis of brines. More particularly it relates to anodes foruse in the electrolytic production of chlorine.

Graphite anodes have been in commercial use in chlorine cells for manyyears despite the unsatisfactory characteristics common to all suchanodes. Generally their wear rates are high and impurities such as COare introduced in the products. In recent years there has been a trendtoward substituting platinum group metals as the anode material.Platinum group metals offer several potential advantages over graphite,for example, lower overvoltage, lower erosion rates, and higher purityof products. Limiting factors are the high cost of the precious metalsand their tendency to short in mercury cells. In order to beeconomically advantageous it is of major importance that the shortingtendency be minimized and the anodes exhibit low overvoltage.

Overvoltage in a chlorine cell can be defined as the voltage in excessof the theoretical needed for chlorine to be produced at the anode at anappreciable rate. Lower chlorine overvoltage is translated into savingsin operating costs through lower power consumption. The theoreticalvoltage can be calculated from standard electrical potentials for thecell. Overvoltage is a property of both the anode material and itsphysical form, e.g. surface roughness, and this must be taken intoconsideration when comparing materials.

The anodes which have platinum group metals as the operative materialmay be composed of platinum group metal in bulk or as a coating on acorrosion resistant substrate. Since it is at the surface of the anodethat the electrolysis occurs, the anode surface is referred to herein asthe operative surface whether the operative material is used in bulk oras a coating. As a practical matter the metal anodes which havepotential commercial value are comprised of a coating of the operativematerial on a suitable substrate. Anodes of this type are well known.The substrate materials are base metals having properties which renderthem corrosion resistant to the environment in electrolysis cells.Examples of suitable 3,691,052 Patented Sept. 12, 1972 corrosionresistant value metals are Ti, Ta, Hg, Zr, W, Al and alloys thereof.These substrate materials are referred to herein as valve metals. It isalso well known to have the valve metal as a layer on a base metal suchas copper which is a good electrical conductor but which corrodes in theenvironment.

In the past it has been proposed to use oxides of platinum group metalsrather than the metals as the operative material of the anode. It hasalso been proposed to use the platinum group metals and oxides such asRuO PtO PdO, in mixtures with various base metal oxides such as oxidesof Ti, Ta, Sn, Bi, Pb, etc. for various purposes such as to increase theadherence of the coating, to reduce the shorting tendency of theplatinum metals, to increase the threshold overvoltage, and to reducethe loss of platinum metals. Another proposal has been to use theplatinum group metal oxides as additives to various oxides such as TiOto introduce semiconductive properties into the TiO in order to make itmore conductive. All these proposed coatings of mixed oxides have infact been mixtures of the oxides. Another approach has been to coat thevalve metal substrate and/or the platinum group metal coating with anoxide for various reasons, for example, toprotect the substrate, makethe platinum metal coating more adherent, and to protect the platinumgroup metal coating from shorting. Lead oxide is among the oxidessuggested for mixture with platinum group metals and their oxides andalso as a coating over a platinum group metal oxide. US. Pat. No.3,213,004, for example, uses an outer lead dioxide surface on a flashcoating of a platinum group metal on a titanium substrate forperoxidation reactions such as the production of sodium perchlorate.British Pat. No. 1,147,442 notes that the threshold overvoltage value ofanodes is increased by the addition of oxides such as lead oxide to aplatinum group metal oxide and suggests the use of a titanium corecoated with a mixture of platinum oxide and lead oxide as an anode forthe preparation of perborates or persulfates.

The proposed anodes have met with varying degrees of success, and therehas been a continuing effort to find more suitable anodes for use inelectrolysis cells.

OBJECTS OF THE INVENTION It is an object of the present invention toprovide metallic anodes with improved physical and electricalcharacteristics. It is another object to provide an anode for theelectrolysis of brines which has chemical and electrical stability, andlow overvoltage characteristics of the platinum group metals. A furtherobject is to provide an anode having an operative surface of leadruthenate or lead iridate. It is still a further object to provide aprocess for the electrolysis of brines which can be effected withrelatively low production costs. These and other objects will becomeobvious from the following description and illustrative examples.

THE INVENTION In accordance with one aspect of this invention a novelanode is provided for the electrolysis of brines said anode having anoperative surface of lead ruthenate or lead iridate. These compounds areparticularly useful for the production of chlorine by the electrolysisof an aqueous solution in that they are chemically inert to theenvironment and exhibit low chlorine overvoltage. A further advantage istheir relatively low cost compared to an equivalent amount of platinumgroup metal. Lead ruthenate, for example, has electrical propertiesequivalent to ruthenium dioxide, but contains only 37.4% Ru by weight.

The lead ruthenate and iridate are chemical compositions having definitecompositions and crystal structures. Lead ruthenate has the chemicalformula Pb Ru O the Pb and Ru being present in a molar ratio of 1:1.Lead ruthenate is not readily attacked by acids or bases, and it is notattacked by molten glass. It does not react with PbO when heated in anintimate mixture at 850 C. Lead iridate can be similarly defined as PbIr O with the Pb and Ir present in a molar ratio of 1:1. The structureand X-ray diffraction pattern of these compounds have been identified inan article entitled Preparation and Properties of Oxygen DeficientPyrochlores by Longo, Raccah, and Goodenough in Materials ResearchBulletin vol. 4 pp. 191-202 (1969).

Of the two compounds lead ruthenate is preferred as the anode material,principally because of its lower cost, and the discussion below will beconfined mainly to lead ruthenate.

The lead ruthenate can be prepared by any known method. For example, itcan be prepared by heating the oxide, carbonate, or nitrate of lead withruthenium powder in air at a temperature in the range of about 600 to1000 C., preferably at a temperature over about 800 C. The leadruthenate is stable at a temperature of 1000 C. Alternatively, andpreferably, it can be prepared from a mixture of PhD and RuO or byco-deposition from salts or resinates. Both of the alternative methodsare shown below. Lead iridate can be similarly prepared.

The lead ruthenate or lead iridate may be used in bulk as the anodematerial or may be used as a coating on a corrosion resistant valvemetal substrate, and it is in accordance with another aspect of thisinvention to provide coatings of lead ruthenate or iridate on suitablesubstrate materials.

In accordance with one method of preparing a coated anode of thisinvention, a mixture of Th0 and Ru0 is heated in air at a temperature inthe range of about 600 to 1000 C., preferably over about 800. Theresultant lead ruthenate is then deposited on a suitable valve metalsubstrate. It can be noted that below about 600 C. the lead ruthenate isnot formed from powders on a practical time scale. Above about 700 C.the valve metals, e.g., Ti or Ta, generally found useful as substratematerials are adversely affected by reaction with oxygen or nitrogen. Byfirst forming the desired chemical compound and then depositing suchcompound on the substrate the anode with a suitable coating can beprepared without damaging the substrate material. Coatings of leadiridate can be similarly formed.

It will be noted that it is difficult to bond the lead ruthenate powderto the substrate material and while the lead ruthenate is the essentialanode material various additives may be used to improve the adherenceand continuity of the coating. It has been found, for example, that leadruthenate powder can be bonded to a valve metal substrate by using a lowmelting glass flux.

Glass fluxes are widely used in the ceramic art. It is common to useprevious metal containing glass fluxes on porcelain for decorativepurposes. Printed circuits of composite previous metal-glass flux filmson alumina ceramics are used as conductors and resistors in theelectronics field. By a flux is meant a glass powder which has been madeby melting together its ingredients. The flux, generally a mixture ofmetal oxide containing glass powders has a melting or softening pointbelow the melting temperature of the principal or operative component ofthe film to which it is added. In use the composite admixture is appliedto a substrate and heated above the melting range of the flux and oncooling it bonds the composite film to the substrate. The flux may beused as a physical mixture of the individual oxide powders, butpreferably the ingredients of the flux are premelted together as ahomogeneous mixture of the powders to form a glass, and the glass isreduced to a fine powder. In either case the flux is melted or remeltedin place. It is also known to add a nucleating agent to the flux, and insuch case the glass will be predominantly crystalline when developed inthe composite film.

Most of the fluxes used in the ceramic art are intended for firing at760 C. to 950 C. and their softening temperature ranges are too high foruse with valve metal substrates such as titanium. In choosing a flux foruse in bonding lead ruthenate or lead iridate films, severalrequirements must be met. The flux must be inert to the corrosiveenvironment of the electrolysis cells, it must have a thermal expansionclose to that of the substrate, it must not react with the leadruthenate, and its melting or softening temperature should be below thatat which the valve metal substrate would be passivated in air. If, forexample, the substrate is titanium, the melting point of the flux shouldbe below 600 0., preferably below 500 C., to permit application of thecoating in air without degradation of the titanium.

A number of glass fluxes have been found which have the combination ofchemical durability, low softening range and thermal expansion neededfor use on valve metal substrates. One group of glasses exists in thesystem, consisting of V 0 plus about 30 to 55 mole percent of PbO, whichmelt at about 600 C. or lower. Within this range compositions containingabout 40 to 52 mol percent are preferred on the basis of lower meltingpoints. The composition containing 50 mol percent PbO is particularlypreferred as it is a eutectic melting at 480 C.

Another type of flux useful for composite lead ruthenate or lead iridatefilms on titanium, is a lead borosilicate glass. Typical of this type ofglass is Pyroceram Brand Cement #89, available from Corning Glass Works.This glass has a softening temperature of about 450 C. and a thermalexpansion of 89 10-' in./in./ C. It contains a nucleating agent and whenheld slightly above its softening temperature for 60 minutes or more,converts to a crystalline material with improvement in chemicaldurability. Still another type of glass suitable for this purpose ismade from lead boratezbismuth subnitrate in the proportion of 2:1 partsby weight.

The amount of flux in composite lead ruthenate or lead iridate films canvary widely, e.g. from about 10% to about 70% glass by weight. Theamount used depends on the density of the flux, and the density of thelead ruthenate or lead iridate powder used in the composition. Toexplain, the glass fluxes are usually made by melting all the startingingredients, and the glass powder obtained from the melt is at its fulldensity. By contrast, the lead ruthenate or lead iridate are preferablypowders made at moderate temperatures by solid state reaction betweenPbO and RuO and IrO Compounds made in this way are of low density andadvantageously have high surface area compared to compounds made abovethe melting point of PhD (888 C.). The low density of lead ruthenate andlead iridate powders are also more economical, since electricalconductivity in composite films is a function of the relative volumes oflead ruthenate or lead iridate and glass. It is convenient to expressthe compositions in parts by weight, but parts by volume are actuallymore important.

Lead ruthenate powder, made at 850 C. as described herein, has anapparent bulk density of 1.2. Glass powder made of the eutecic PbO-V Ocomposition by melting the starting materials at 900 C. has an apparentbulk density of 1.5, but when melted for the deposition of the film,e.g. at about 500 to 600 C., the flux goes to full density of 5.2. Thedensity of the Pb Ru O remains at 1.2. A fired film which contains about60% flux by weight has only about 25% flux by volume.

Another method of developing a lead ruthenate coating on a substrate isby deposit from a solution of salts or resinates of ruthenium and lead,e.g. ruthenium chloride and lead acetate dissolved in a mixture ofglycerin and isopropyl alcohol, or a solution of ruthenium resinate andlead resinate. In order to form the compound Pb Ru O from solution, thecoating must be heated in an oxidizing atmosphere at a temperature inthe range of about 300 to 1000 C., typically about 450 C. Below about300 C. the compound is not formed. Thin films of lead ruthenatedeposited in this way and fired at 450-550" C. have good adherence tovalve metal substrates such as titanium.

Evaluation of bulk material for use as anode coatings on titaniumiscomplicated since many refractory materials, when used in the pure form,do not adhere well when fired on titanium at practical temperatures. Toallow comparison of coating materials, it is convenient to use them aspowders suspended in a resin solution. After application, this paint isdried at the minimum temperature necessary to remove the vehicle. Atoluene solution of an acrylic resin is a suitable medium for thispurpose. It can be dried at 70 C., and at this low temperature, thematerials being evaluated obviously do not .undergo solid statereactions with titanium. An example is given below of overvoltagemeasurements made on coatings of RuO Pb RuO O and Bi Ru O prepared inthis way.

It is of interest that bismuth ruthenate, having the chemical formula BiRu O and having many properties similar to Pb Ru O was found unsuitableas an anode material for the electrolysis of brine, due to high chlorineovervoltage.

EXAMPLE 1 Equimolecular amounts of PhD and Ru powders were mixedtogether and heated in air at 850 C. for 50 hours. A black powder ofuniform color and appearance was formed. The X-ray diffraction of theresultant material was determined and found to match the pattern knownfor Pb Ru O The X-ray diifraction pattern of the material prepared andthe known pattern for Pb Ru O are given in Table I. The known values ofPb Ru O are taken from the above-mentioned article by Longo et al.

In Table I I I, represents a ratio of all intensities to the highestintensity and hkl data are Miller indices. A comparison of these valuesshows that the pattern of the prepared sample matched closely with thevalues given in the literature.

As a further check on the prepared sample, the X-ray diffraction patternof the prepared sample of Pb Ru O was matched agamst the ASTM values forRuO and PhD.

Companson 1s given in Table II. In this Table dA. represents the Braggspacmgs.

TABLE I X-ray Diffraction Patterns of PbzRuzOo Compared Sample preparedLiterature M1 hkl I/I1 hkl 0 0.1 0 553,731 1.5 553,731 0.8 800 733 1.2733 822,000 0.2 822,000 751,222 20.9 555,751,002 840 18.7 840 753,911911,753 004 0.1 004' 931 0.0 931 844 10.9 s44,755,933,771 itt??? 1971"35175913575" 00 1,022,050 0.3 053 1,111,775 0.3 1,111,775 880 0.0 9301,131,971,955 1.1 1,131,971,955 0 973,1,133,1,002 28.2 973.1,133,1,0024O 4, 1,200, 18.9 1,200,834

TABLE 11 PbzRmOe (111. I l, 11111 dA A comparison of the values in TableII shows that the prepared lead ruthenate was not a mixture of R110 andPbO melts at 880 C. A sample of the prepared powder tal structure.

In further tests on the prepared powder it was heated to 1000 C. in airand there was no loss in weight and no melting, further evidence ofcompound formation since 'PbO melts at 80 C. A sample of the preparedpowder was boiled for one hour in hydrochloric acid and no Pb dissolved.The latter test is an indication of the inert nature of the compound.

EXAMPLE 2 A sample of the lead ruthenate powder, prepared as describedin Example 1, was pressed at 9000 kg./sq. cm. into a wafer 2 mm. thick.The wafer had a circular area of 1 sq. cm. The wafer was then sinteredin air overnight at 1000 C. The resultant material was used as an anodein a laboratory scale cell for the electrolysis of sodium chloride andfound to have a low chlorine overvoltage, equivalent to that for bulkRuO As noted above, the Pb Ru O is less expensive than RuO since itcontains only 37.4% of the expensive RuO by weight.

EXAMPLE 3 A solution containing Ru and Pb in molar ratio of 1:1 was madeand used to form thin films of Pb Ru O on titanium. This solutioncontained the following parts by weight:

Ruthenium resinate dissolved in a mixture of oil or rosemary,nitrobenzene and chloroform (4% The mixture of essential oils was chosento solubilize the two resinates in a solution having a viscosity suitedto application by brushing. A solution of this concentration gives afired film about 1000 angstroms in thickness for each application.

The above solution was used to prepare thin films of Pb Ru O on titaniumsheets and on glass microscope slides, by firing in air to 450 C. with aminute soak at peak temperature. A comparative set of thin RuO films wasprepared on these substrates on the same firing cycle by brushapplication of a solution of RuCl in isopropyl alcohol. The presence ofPb Ru O was determined by measuring electrical resistivity of the filmon glass, at room temperature and at liquid nitrogen temperature. Thetemperature coefiicient of resistance was positive and of the same orderas that reported for bulk Pb Ru O in the Longo, Raccah and Goodenoughreference cited above. In contrast, the thin film of RuO- on glass hadthe same electrical resistivity at the two temperatures, i.e. its TCRwas zero.

The films on titanium were tested as anodes in a brine cell (22% NaCl)at 20 C. at a pH of 3.7. Overvoltage was measured in the usual way usinga Luggin probe connected to a normal calomel electrode. Values for thetwo coatings were:

POTENTIAL VOLTS It can be seen that the superiority of Pb Ru O ismostpronounced at the higher current densities, the range of practicalcommercial importance in chlorine production.

EXAMPLE 4 To prepare bismuth ruthenate, powdered Bi O and R1102, inratio of 1 mole:2 moles, were mixed thoroughly, the mixture was placedin a porcelain dish and heated in air at 750 C. for one hour.

Upon cooling, the product was a black very conductive powder.Microscopic examination showed that the intense orange color of thestarting Bi O had completely disappeared.

The bismuth ruthenate powder was sifted and the fraction which passed a325 mesh sieve was made into a paint using a toluene solution of anacrylic resin. Proportions were chosen to give a film on titaniumcontaining parts by weight BigRuzoq and 5 parts resin after removal ofthe toluene by baking at 70 C. Paints containing RuO and Pb Ru Oprepared in the same way were applied to titanium and baked at 70 C.

The three coating materials were tested at 20 C. in a cell containing asolution containing 22% NaCl at a pH of 3.7. At a current density ofamps/square foot, bismuth ruthenate samples had an overvoltage potentialof more than 3 volts with reference to a normal calomel electrode. Incontrast, RuO samples and Pb Ru O had potentials of 1.2 volts at thiscurrent density.

This comparison shows that Bi Ru O is not suitable for the production ofchlorine by the electrolysis of brine.

EXAMPLE 5 A composite film, about one mil thick, was prepared ontitanium from a mixture of lead ruthenate and a flux. The flux waschosen from the PhD-V 0 system and the composition was equimolar, theeutectic with a melting point of 480 C. It was made from the oxides at900 C. On cooling to room temperature it did not crystallize. It wasground to a powder to pass through a 325 mesh sieve, applied to titaniumand heated to 500 C. On cooling, it was observed that the powder hadmelted and was firmly bonded to titanium, indicating that the thermalexpansion is close to that of titanium. The glass coating was anelectrical insulator. It did not generate chlorine when tested as anodein a brine cell. The glass showed no apparent chemical attack from thebrine.

The composite film was formed by mixing lead ruthenate powder preparedas in Example 1 and the PhD-V 0 fiux (325 mesh), in proportion of 1:2 byweight. The mixture was suspended in a mixture of essential oils andapplied to titanium by brushing. The coated titanium was fired in air to500 C. with a 20 minute soak. On cooling an adherent, highly conductivecomposite film about one mil thick, was obtained. This coating wastested under the conditions described in Example 3 and had these values:

Potential volts Amps/sq. ft.: Pb Ru o zflux 2 100 1.18

a 1 2, by weight.

The overvoltage of this composite film is not as low as that of thethin, pure Pb Ru O film described in Example 3. In some applications,the overvoltage of the composite film is acceptable because of thedurability and increased operating time obtainable with this relativelythick, very adherent film.

EXAMPLE 6 A solution containing Ir and Pb in a molar ratio of 1:1 wasmade and used to form a thin film of Pb Ir O on titanium. This solutioncontained the following parts by weight:

Iridium resinate dissolved in a mixture of oil of rosemary, nitrobenzeneand chloroform (6% Ir) 6.23 Lead resinate dissolved in a mixture of oilof rosemary, nitrobenzene and chloroform (27.8%

PB) 1.45 Oil of lavender 0.44 Oil of petitgrain 0.44 Oil of camphor .44Rosin dissolved in oil of spike (50% rosin 1.00

POTENTIAL VOLTS Amps/sq. ft 100 200 400 600 800 l, 000

This demonstrates the low chlorine overvoltage of the lead iridatecoated samples.

EXAMPLE 7 Lead iridate was made by mixing equimolar amounts of IrO andPhD powders, minus 325 mesh. The mixture, green yellow in color, wasplaced in a porcelain crucible and heated in air to 800 C. It was heldat 800 C. for 16 hours and cooled gradually. The product was a flufiy,uniformly black powder which was highly conductive. This was identifiedby X-ray diffraction as Pb Ir O with a structure similar to leadruthenate, described above.

Lead iridate powder, -325 mesh, was mixed with eutectic PhD-V 0 glasspowder, also 325 mesh, in the ratio of Pb Ir O :glass, 2:3 by weight.After thorough dry mixing in a mechanical shaker, the mixed powders weresuspended in a vehicle made by dissolving rosin in a mixture ofessential oils. The vehicle contained 16.6%

POTENTIAL VOLIS Amps/sq. ft 100 200 400 600 800 1, 000

PbzIrzOa plus flux 1. 24 1. 26 1. 29 1. 36 1. 45 1. Do. 1. 22 1. 25 1.29 1. 31 1. 37 1. 48

1 Sample 1. 2 Sample 2.

What is claimed is:

1. An anode for the electrolysis of brines comprising a substrate of acorrosion-resistant valve metal selected from the group consisting oftitanium, tantalum, hafnium, zirconium, tungsten, aluminum and alloysthereof, said substrate having an electrically conductive coatingconsisting essentially of Pb Ru O or Pb lr o 2. An anode of claim 1wherein the coating consists essentially of Pb 'Ru O 3. An anode ofclaim 1 wherein the coating consists essentially of Pb Ir O 4. An anodeof claim 1 wherein the electrically conductive coating contains from 10%to 70% by weight of an inert, nonconducting glass flux to bond thecoating to the substrate.

5. An anode of claim 4 wherein the flux is composed of V 0 and 30 to molpercent PbO.

6. An anode of claim 5 wherein the flux is a eutectic mixture of PbO andV 0 References Cited UNITED STATES PATENTS 3,528,857 9/ 1970 Lieb et a1204-290 R FOREIGN PATENTS 1,195,871 6/ 1970 Great Britain.

6,606,302 11/1966 Netherlands.

JOHN H. MACK, Primary Examiner R. I. FAY, Assistant Examiner U.S. Cl.X.R. 117-230; 20429l

